1. Field of the Invention
The invention relates to the field of fluid control valves. More particularly, the invention relates to fluid control valves for regulating supersonic gas flow.
2. Description of the Prior Art
The purpose of a supersonic flow valve, for example, a thruster valve in a rocket booster or divert thruster, is to align the momentum of fluid flow with the direction of the desired thrust. During optimal performance, all the velocity components of the fluid flow are aligned with the direction of flight or thrust. Vectored flow, that is, flow that has velocity components at an angle relative to the direction of flight, will produce a vectored thrust. Also, if the flow is not properly aligned and turbulence or flow separation occurs, the separation will dissipate kinetic energy in the flow, thus reducing the velocity and the amount of thrust obtained from the fuel.
Conventional valves used to control supersonic fluid flow typically have a cone-shaped pintle, typically a 45-degree cone, that seats in the orifice of the nozzle. When the valve closes, the pintle stops flow through the orifice; when it opens, the pintle not only allows fluid to flow into the nozzle, but also provides a boundary surface that serves to align the velocity components of the fluid flow. Depending on the particular application, the boundary surface is shaped to guide the flow toward the nozzle exit, with the least amount of losses due to friction and/or flow separation.
Several disadvantages are inherent with the use of a 45-degree pintle in applications of supersonic fluid flow. First and foremost is the problem of material ablation in such pintles as a result of rapid heat transfer from the fluid to the tip of the pintle. FIGS. 1A–3C listed below are illustrations of the typical construction and flow conditions within a conventional 45-degree pintle valve. FIG. 1A is a perspective view and FIG. 1B a cross-sectional view of the convention 45-degree valve, showing the pintle and the nozzle. FIGS. 2A–3C are color graphs of computational fluid dynamics analyses of fluid flow, whereby it is noted that the pintle is shown facing upward instead of downward as in FIGS. 1A, 1B, and that fluid flow through the nozzle is toward the upper end of the graph. FIG. 2A illustrates temperature (T) conditions, FIG. 2B illustrates presssure (P) conditions, and FIG. 2C illustrates Mach (M) conditions in the valve during fluid flow.
Color and color gradations are indicators of a change in the respective P, T, and M conditions, with red representing a maximum condition, and dark blue a minimum condition. A shock wave is indicated by the color yellow. In FIGS. 2A and 2B, the 45-degree pintle valve is shown fully open, providing a throat in the area between the pintle and the inlet to the nozzle. A shock wave arises as a result of the rapid expansion of the fluid as it flows into the nozzle. FIGS. 2A and 2B show two areas of high pressure and high temperature, respectively. The first area of high temperature/high pressure is located in the plenum of the valve and ends at the edge of the nozzle inlet. This first area contributes largely to the high heat flux in the nozzle and leads to a high rate of material ablation and erosion at the nozzle inlet. The second area is situated directly at the tip of the pintle. This area is particularly problematic due to the high surface to volume ratio of material at the tip of the pintle to the amount of heat being transferred from the fluid to the pintle. Because of this, the material at the tip rapidly melts away, thereby drastically changing the geometry of the flow control surface. Any change in geometry results in a rapid deterioration of thruster performance. In some conventional valves, the tip of the pintle and the nozzle inlet are coated with rhenium, a metal capable of withstanding the high temperatures generated in such a valve. Although rhenium can minimize the material ablation in the valve, it has disadvantages in that it is an extremely heavy material and also very expensive and difficult to obtain.
Accurate alignment of the 45-degree pintle and nozzle orifice is a critical aspect of good performance of the conventional 45-degree pintle valve in supersonic flow control applications. Any misalignment of the pintle leads to significant performance degradation. For example, if the pintle is off-center, flow will be greater on one side of the orifice, resulting in a higher pressure on that side. The gas under the higher pressure will tend to flow into areas of lower pressure, causing secondary flow fields with their inherent energy losses due to additional heating and separation. Furthermore, vibrational excitations emanating from the structure/vehicle may also cause a misalignment of the pintle, which, if the pintle and nozzle touch, may result in catastrophic failure of the valve.
The fact that the 45-degree pintle is a rigid mass situated in the flow path is a further disadvantage. The fluid flow is forced along the boundary surface of the pintle as it passes through the throat, resulting in loss of kinetic energy in the flow due to friction against the boundary surface. Also, gaseous fluids are often contaminated with solid particles. These solid particles, being heavier than the gas particles, collide with the pintle and nozzle, abrading and eroding the boundary surface and exacerbating the material ablation resulting from heat transfer.
There are many applications for booster or thruster valves in which it is critically important that the fluid flow be precisely controlled. One example is the use of fluid flow valves in missile defense systems in which a seeker component of a missile system locks onto the flight path of an in-coming hostile missile, with the purpose of intercepting the flight path and destroying the missile. Currently, the valve technology in such guidance and seeker systems uses the traditional pintle design and nozzle. Typically, these valves operate in the full-on and full-off positions, and consequently, are pulsed as a means of precisely controlling the mass flow of the hot gases and, thus, the thrust. In some applications, these cone pintles are easily operable in an on-off mode because the stroke length (actuation length) is so long. Due to the severe weight and space constraints that are typically applied to the construction of vehicles in a missile system, operating a thruster valve in pulse-mode has disadvantages that affect accuracy and cost of the missile system. For example, the pulsing produces significant structural vibrations that severely hamper the operation and performance of the high-precision components in the guidance and seeker systems on the vehicle. Consequently, the seeker housing has to be vibrationally isolated from the body of the vehicle in order to maintain proper performance, and this is technically a difficult and costly task.
Another type of known valve includes a pintle that is very slightly angled, about 2°25′, rather than having the 45-degree type pintle discussed above. U.S. Pat. No. 3,975,116 (Feild; issued 1976) teaches such a valve for hydraulic operations. A severe disadvantage of the Feild device for supersonic flow applications is that it teaches a straight-edge nozzle inlet (orifice), that is, a 90° drop-off at the edge of the orifice. The rate of expansion is uncontrolled with such a sudden drop-off, making the device unsuitable for use with supersonic fluid flow.
What is needed, therefore, is a fluid control valve that provides enhanced valve performance and a longer operational life. What is further needed is such a valve that reduces heat transfer loads in the nozzle throat region, reduces kinetic energy losses, and optimizes the flow path. What is yet further needed is such a control valve that minimizes response time and reduces the power required to actuate the valve. What is still yet further needed is such a device that reduces vibrational stress on surrounding system components.